1. Field of the Invention
The present invention relates to a viscoelastic resin composition for vibration damping material and more particularly to a viscoelastic resin composition for vibration-damping material to form an intermediate layer in the laminate structural member of machines, buildings, and vehicles.
2. Description of the Prior Art
In these days of well-developed traffic facilities and disordered urban environments where dwelling houses are close to factories, noise and vibration constitute a social problem as public nuisance. Noise and vibration also aggravate the working environment. Attempts are being made to reduce noise and vibration by providing the metal material as a source of noise and vibration with the vibration-damping function to absorb vibration energy and convert it into thermal energy, thereby damping the vibration velocity or vibration amplitude and reducing the acoustic radiation.
To improve the vibration-damping function, there has been proposed a composite vibration-damping material of laminate structure which is composed of metal layers and a viscoelastic intermediate layer interposed between them. It is now in use as a structural member for automotive engine cover and oil pan, hopper chute, conveyor stopper, household electric appliance, metal working machine, and precision machine, which are required to generate as little vibration as possible.
In general, the composite vibration-damping material mentioned above depends for its performance on the viscoelastic intermediate layer. The vibration-damping performance is expressed in terms of loss factor (which indicates a degree of conversion of external vibration energy into thermal energy by internal friction, or an amount of mechanical hysteresis loss caused by vibration). The loss factor has a peak value at a certain temperature, and the vibration-damping material can be used most effectively in the neighborhood of that temperature.
According to the known prior art technology, the conventional composite vibration-damping material has a viscoelastic intermediate layer which is made of polyester alone (*1), polyester incorporated with a plasticizer (*2), polyester incorporated with an organic peroxide (*3, 4), a plurality of polyesters in combination (*5, *6), polyurethane foam alone (*7), polyamide alone (*8), ethylene-vinyl acetate copolymer (*9), polyvinyl butyral or polyvinyl butyral-polyvinyl acetate composition incorporated with a plasticizer and tackifier (*10), copolymer of isocyanate prepolymer and vinyl monomer (*11), or a specific copolymer (*12, *13, *14). FNT *1Japanese Patent Kokai No. 143880/1975 FNT *2 Japanese Patent Kokai No. 93770/1976 FNT *3 Japanese Patent Kokai No. 41080/1976 FNT *4 Japanese Patent Kokai No. 83640/1976 FNT *5 Japanese Patent Kokai No. 295949/1987 FNT *6 Japanese Patent Kokai No. 202446/1988 FNT *7 Japanese Patent Kokai No. 91981/1976 FNT *8 Japanese Patent Kokai No. 159160/1981 FNT *9 Japanese Patent Kokai No. 34949/1982 FNT *10 Japanese Patent Kokai No. 27975/1980 FNT *11 Japanese Patent Publication No. 27975/1980 FNT *12 Japanese Patent Kokai No. 258262/1985 FNT *13 Japanese Patent Publication No. 12451/1964 FNT *14 Japanese Patent Publication No. 34703/1970
The composite vibration-damping material is required to have a high loss factor and a high adhesive strength between the viscoelastic intermediate layer and the metal layer. However, these requirements have not been fully satisfied. For example, vibration-damping performance and adhesive strength do not consist together at room temperature in the case where the viscoelastic intermediate layer is made of polyester resin (as in Japanese Patent Kokai Nos. 143880/1975, 93770/1976, 41080/1976, and 83640/1976), or even in the case where the viscoelastic intermediate layer is made of a plurality of polyesters in combination (which differ in glass transition point) so that it produces its vibration-damping effect over a broad range of temperature (as in Japanese Patent Kokai Nos. 295949/1987 and 202446/1988).
Moreover, the composite vibration-damping material having a viscoelastic intermediate layer should be capable of fabrication such as pressing and bending. However, conventional ones do not meet this requirement; they are liable to wrinkling and cracking during fabrication and also to peeling between the viscoelastic intermediate layer and the metal layer during heat treatment (such as baking finish) which follows fabrication.